Conventionally, various joining methods have been used in apparatuses for joining objects to each other. As one of the methods for joining objects having metallic portions to each other at a comparatively low temperature, there is a known method in which the metallic portions are subjected to a cleaning process such as removal of adsorbent substances, activation of surfaces or the like by applying plasma to the metallic portions before the metallic portions are joined to each other.
For example, Japanese unexamined patent publication No. 2001-60602 A proposes a technique that, in an apparatus for mounting electronic components onto circuit boards such as printed boards, an electrode of the circuit board is cleaned by applying plasma thereto, and thereafter joined to an electrode of the electronic component. Also, Japanese unexamined patent publication No. 2002-368039 A discloses a technique that gold stud bumps formed on aluminum electrodes of two IC chips are subjected to a cleaning process with plasma, and after removal of contaminations of the bump surfaces, their bumps are pressurized and joined to each other in a heated state.